We have recently observed that certain nonphagocytic, nonadherent lymphocyte-like cells present in human peripheral blood which lack the usual T-cell and B-cell markers, can be activated to secrete mediators of cellular inflammation, lymphokines, upon exposure to immmune complexes or heat-aggregated gamma globulin. This has been shown to be a consequence of interaction of the Fc portion of activated immunoglobulin G molecules with Fc receptors on the surfaces of the active cells. These effector cells are either identical to or very closely associated with the group of cells which mediate antibody-dependent cell-mediated cytotoxicity (ADCC). Fc receptor-bearing T lymphocytes are also active in both phenomena. The major lymphokine activity studied has been leukocyte migration inhibitory factor (LIF). It is hypothesized that lymphokine production induced by exposure of FC receptor-bearing lymphocytes to circulating or tissue-bound immune complexes plays a significant role in some protective and injurious inflammatory processes. Under this project preliminary experiments will be performed to test the hypothesis while additional basic information is gathered from in vitro studies of the interaction of subclasses of human lymphocytes with immmune complexes. The following are among the topics to be studied. The characteristics of the lymphokine-producing cells, the frequency and/or activity of such cells in health and immunodeficiency, the types of lymphokines produced in terms of identity or lack thereof with those produced by antigen or mitogen-stimulated T lymphocytes,the optimal and minimal conditions for lymphokine production, other manifestations of lymphocyte activation by interaction with immune complexes, the characteristics of the stimulatory complexes and fragments, the nature of their interaction with effector cells and the time course of those interactions.